This invention is related to an apparatus for recording X-ray images. An apparatus of this type is known for instance from U.S. Pat. No. 3,069,551.
The known devices, which operate as image intensifiers with an image recording and reproducing system have the purpose of providing an improved electrical installation for generating a high lighted visualization of an image which is derived from or produced by a radiation source. Such an image intensifier consists or an image recording system which can deliver an electric pulse sequence corresponding to the image, and of an image reproducing system which transforms these pulses into an image. The latter system corresponds to a large extent with the television reproduction devices common today.
The recording system consists of two panel type radiation-transmitting electrodes and, in direct contact with them, two solid layers in intimate area contact with each other, one layer consisting of a photoconductive substance while the second layer consists of an electrically insulating substance which is able to store electric charges, and where the photoconductive layer is impinged upon both by the rays of the image to be intensified and by rays of a light source scanning the entire image area optically according to a specified raster. But such devices have not been successful because charge storage at an interface is still difficult if it is to operate with the necessary homogeneity and reproducibility. In addition, a large scanning time is required, so that with the known arrangement pictures of moving organs can hardly be obtained.
More recently proposals have appeared according to which layers consisting of phosphorescent materials are to be used as storage elements. That is, luminophors are to be used which, when exposed to high-energy radiation, are excited as a function of the intensity of these rays, and as such are suitable for storing this excitation. Such arrangements are described e.g. in U.S. Pat. Nos. 3,859,527 and 3,975,637. Both methods are based on utilizing the effect of the light-stimulated luminescence, i.e. transforming the X-ray light by way of a luminescent screen into visible light first. The brightness-modulated image is then stored in the form of excited electron states in traps in a connected or in the same phosphor. The optic-electrical reading or conversion occurs thereafter by local scanning of the phosphor with a luminous spot through the effect of light-stimulated or thermo-stimulated "exposure". The emitted light is transformed by an optoelectric detector into electrical signals, which can be visualized in the manner common in television. A disadvantage here is that, apart from other losses, the storage effect itself reaches an efficiency of at most 10%.
The scanning--to be carried out in a raster system--of the total area of the recording medium by means of a luminous spot requires, however, a free space in front of the phosphor layer. For this reason the optoelectric detector or a preceding focusing lens must be arranged at a great distance from the light-emitting area of the phosphor. Thus only a small portion of the light intensity activated during scanning is picked up by the optoelectric detector. This leads additionally to an undesirably small signal to noise ratio and to a small dynamic range.